The employer will see that you “could” be doing more work, since you accomplish everything in 4 hours. “You don’t have enough work to occupy your time”, they’d say in my country.
That’s why people act busy. Because when you’re efficient, you get punished with more work.
92% of young people would sacrifice other perks for a 4-day workweek—here's what they'd give up ( www.cnbc.com )